One of the more common applications for a sensor capable of detecting gas bubbles in a liquid arises in the field of medicine, in connection with pumps used to infuse liquids into a patient. Most intravenous (IV) pumps employ an air bubble sensor to monitor the IV lines carrying liquid from the pump to a patient, to detect air bubbles. The air bubble sensors respond to bubbles larger than a predetermined maximum size, but also detect smaller air bubbles. Even relatively small air bubbles can create a problem if their density or total volume is sufficient to produce a potentially life threatening air embolism should the small bubbles combine within the patient's cardiovascular system to form a large air bubble.
An air bubble sensor suitable to detect bubbles in an IV line typically includes two piezoelectric crystals that are mounted in a housing on each side of a slot. The IV line carrying liquid to the patient is inserted into the slot so that it contacts the inner surfaces of the housing on each side of the slot. One of the piezoelectric crystals is electrically excited at its resonant frequency to produce ultrasonic sound waves that are directed transversely through the IV line toward the other piezoelectric crystal. The other crystal resonates at approximately the same ultrasonic frequency as the transmitting crystal, and in response to the ultrasonic sound waves that it receives, produces a corresponding electrical output signal. Since liquid is a much better conductor of ultrasonic sound than is air, any gaseous bubbles entrained in the liquid that flows through the IV line between the transmitting and receiving piezoelectric crystals will attenuate the ultrasonic sound waves reaching the receiving crystal in a manner indicative of gas bubble size and density. Changes in the magnitude of the ultrasonic sound waves received produce a corresponding change in the electrical signal output from the receiving crystal. A monitoring circuit is coupled to the receiving crystal and responds to the electrical output signal. This monitoring circuit typically stops the pump and produces an audible and/or visual alarm if either too many small gas bubbles or a gas bubble that is larger than a predetermined maximum is detected.
A prior art air bubble sensor used by the assignee of the present invention employs a housing comprising two plastic halves--one half supporting the piezoelectric crystal that generates the ultrasonic sound waves and the other half supporting the piezoelectric crystal that receives the ultrasonic sound waves after they have passed through the sensor cavity or slot. When assembled, the two halves of the housing define a slot of an appropriate width to receive an IV line, with the transmitting crystal mounted within one half of the housing on one side of the slot and the receiving crystal mounted opposite, within the other half of the housing. Care must be taken to ensure that the transmitting piezoelectric crystal is properly aligned with the receiving crystal when the two halves of the sensor are assembled. Small errors in the alignment can adversely affect the performance of the sensor.
In this prior art sensor, the steps involved in mounting each piezo element within its respective half of the housing are relatively involved, time consuming, and labor intensive. Specifically, each piezoelectric crystal used in the sensor is first mounted on a separate alumina substrate, which serves as a carrier. Before the crystals are mounted on their respective carriers, a pattern of electrically conductive paths and pads of palladium-silver alloy are applied to the alumina substrate, e.g., by silk screening. A piezoelectric crystal is then affixed to the alumina substrate using a conductive epoxy that electrically connects the downwardly facing surface of the crystal to one of the conductive paths on the alumina substrate. One end of a thin wire (actually two thin wires in parallel are preferably used to improve reliability) is then ultrasonically welded to a conductive pad applied to the alumina substrate; the other end of the wire is mechanically bonded to the outwardly facing surface of the piezoelectric crystal, with a drop of conductive epoxy added to reinforce the bond. The alumina substrate with the piezoelectric crystal mounted on it is then installed in a cavity within one of the two plastic housing halves, and conductive epoxy is used to connect the conductive paths on the alumina substrate to copper pins that extend externally of the housing. A non-conductive epoxy is used as a potting agent to cover the alumina substrate and copper pins, making an airtight seal over the assembly within the cavity. After each substrate mounted upon its carrier is thus installed within a cavity formed within one of the housing halves, the two plastic sections of the housing are assembled to form the air bubble sensor.
Clearly, it would be preferable to employ a system and method for mounting a piezoelectric crystal that is simpler than the prior art technique described above. It would be desirable to mold a single integral housing to support both the transmitting piezoelectric crystal and the receiving piezoelectric crystal. However, it is essential that any simpler mounting system not degrade the reliability of the air bubble sensor or reduce its sensitivity. An intermittent electrical connection between one of the piezoelectric crystals and the electrical signal path in the sensor or a loosened mount of the crystal could have potentially life threatening consequences if the air bubble sensor should fail to detect air bubbles. Any simplified system for mounting a piezoelectric crystal in an air bubble sensor must therefore ensure that the crystal remains firmly seated within its support and must ensure that a good electrical connection is maintained between the crystal and the external circuitry.